So far polyethylene is still one of the most important materials for manufacturing articles in various shapes by extrusion molding, blow molding, injection molding, film molding, rotational molding, or other molding methods. Typical examples of articles of this kind include tube, drum, bottle, box, film, piece, etc. However, polyethylene materials having different physical properties and processabilities are required for various articles, so as to obtain qualified ones. Some special articles, for example, tube, bottle and film, require the polyethylene having not only excellent physical property and processability, but also a good balance between the physical property and the processability. Stringent requirements on the polyethylene material greatly increase difficulty in microstructure design of the polyethylene material, especially in material composition and molecular structure design.
Obviously, in all the polyethylene articles, the molding of a large and hollow article is the most difficult, particularly for the rotational molding, because the rotational molding requires a very good balance between the physical properties and the processability of a polyethylene composition. However, a lot of known polyethylene raw materials, in particular conventional crosslinked polyethylene compositions, cannot completely meet the abovementioned stringent requirements. Generally speaking, the design of the molecular structure of polyethylene determines its specific use and way of formation, because the molecular structure is immediately related to physical and mechanical properties as well as processability of the material.
As is known to all, for large and hollow articles, the best way for molding is rotational molding, because the hollow articles resulting from rotational molding are integrate and have no weld lines or joints, which, without doubt, is conducive to improvement of the physical properties of the articles. Usually, the rotational molding method comprises the following steps: (1) feeding the plastic resin powders into a hollow die; (2) heating the hollow die to melt the resin powders therein, (3) rotating the hollow die so as to take advantage of the centrifugal force to make the melted resin powders stick closely to the inner wall of the rotating hollow die, thereby achieving the molding and self-densifying; (4) cooling the hollow die and the shaped resin article therein by injecting such cooling mediums as air and/or water; (5) withdrawing the molded article from the cooled hollow die. For the details of the abovementioned rotational molding, reference can be made to “ROTATIONAL MOLDING TECHNOLOGY,” 2001, pp. 2-13, as well as U.S. Pat. No. 4,029,729 and U.S. Pat. No. 4,115,508.
Rotational molding crosslinked resin articles have a series of crucial advantages, e.g., higher heat distortion temperature (HDT), higher glass transition temperature, higher strength of extension, lower heat expansion coefficient, better environmental stress cracking resistance (ESCR), good weather resistance, prominent chemical solvent resistance, etc.
A plurality of polymer resins, such as polyethylene, polycarbonate, polyamide, polyvinyl chloride and the like, can be formed by rotational molding, wherein polyethylene articles in various forms take up more than 85% of the market shares owning to their low costs, excellent processability, and good chemical erosion resistance. However, conventional polyethylene articles may be impaired to some extent by hydrocarbon, gasoline and other chemical substances, especially by esterification and/or halogenated hydrocarbon. Moreover, a poor creep resistance is another defect that limits the final use of polyethylene products. When chemical storage tanks or large or unsupported chemical storage containers need to be prepared, the occurrence of crosslinking is usually required during the formation of high-density polyethylene composition which is used for preparing articles of this kind, so as to improve properties of the final articles.
Molecular weight of a formed composition attaches great importance to processing flowability as well as overall mechanical properties of a crosslinked formed article. Polyethylene having a high flowability with the introduction of organic peroxide crosslinking agents has been reported. U.S. Pat. No. 3,876,613, U.S. Pat. No. 5,367,025, U.S. Pat. No. 4,267,080 and U.S. Pat. No. 4,029,729 disclose some crosslinked polyethylene compositions for rotational molding. In the above patent literatures, the polyethylene compositions for rotational molding are polyethylene homopolymers or copolymers having a melt index of at least 10 g/10 min, for example 20 g/10 min. However, as is known to all, the higher the melt index of the polyethylene composition is, the lower the crosslinking efficiency is, and the poorer the impact resistance and environmental stress cracking resistance (ESCR) of the final article are.
A high-density polyethylene is still greatly expected, so that a final article can be provided with a chemical erosion resistance, a tenacity and a soften temperature as high as possible. For example, U.S. Pat. No. 3,876,613 discloses a crosslinked polyethylene composition having a density in a range of 0.920-0.970 g/cm3, preferably in a range of 0.940-0.970 g/cm3, the chemical erosion resistance of which is extraordinary.
A crosslinking agent is usually used, enabling the occurrence of crosslinking of plastic resins, such as polyethylene compositions. An ideal crosslinking agent shall enable a polyethylene composition to have an improved crosslinking retardation time (scorch time) so as to avoid impact on formation by premature crosslinking of the composition, and to increase processability of the composition.
Crosslinking agents commonly used for crosslinking of rotational molding polyethylene compositions are various organic peroxides. However, the decomposition of most peroxide crosslinking agents begins before the temperature of about 150° C., and ends at the temperature of about 180° C. The decomposition temperature of the above peroxide crosslinking agents is so low that premature crosslinking happens before a composition formed by the crosslinked polyethylene becomes compact or during its formation, hence, the effect of the crosslinking agents is unsatisfactory.
To resolve the technical problem of premature crosslinking of the rotational molding polyethylene composition, one solution is to add an aid to a crosslinked polyethylene composition using a peroxide as crosslinking agent to improve the crosslinking retardation time. U.S. Pat. No. 7,842,759, U.S. Pat. No. 7,056,967 and U.S. Pat. No. 6,864,323 disclose several free radical inhibitors, but the polyethylene compositions disclosed thereby are polyethylene composition for use in wires or cables, having extremely low melt indexes, for example, lower than 1.0 g/10 min. For instance, nitroxides and 1,1-diphenylethene are used in these reference documents as free radical inhibitors for delaying crosslinking, and the resin compositions using these inhibitors are formed by extrusion to be used for preparation of wires, cables and tubes.
It is known that hydroquinone and antioxidants can be used as crosslinking retarders for crosslinked resin compositions which employ peroxides as crosslinking agents. However, the addition of hydroquinone and antioxidants deteriorate δ(delta) torque value, which make the final properties of the cured article worse. As to the technical details hereinabove, reference can be made to U.S. Pat. No. 5,292,791 and U.S. Pat. No. 5,245,084.
Another problem that arises from introduction of organic peroxide crosslinking agents to polyethylene having a high flowability is that the decomposition of these organic peroxides leads to formation of free radicals having low molecular weights, thereby forming substances with low boiling points by capturing hydrogen from polyethylene. Bubbles are formed in the finished article due to these substances with low boiling points, and irritating smell is released therefrom.
To avoid formation of the above bubbles, JP18296/1979 (published) suggests use of a crosslinked polyethylene composition with the introduction of an ethinylated unsaturated peroxide and diene polymer. U.S. Pat. No. 4,267,080 discloses a crosslinking aid for avoiding bubble formation and improving demolding property, which is selected from a group consisting of 1,2-butadiene, triallyl cyanurate (TAC), and/or triallyl isocyanurate (TAIC).
The disclosures of all above-mentioned reference documents are incorporated herein in entirety by references.
So far, no reference document has disclosed such technical content concerning using a crosslinking agent, a crosslinking promoter and a free radical inhibitor simultaneously in a rotational molding polyethylene composition, so as to prevent premature crosslinking during the rotational molding of the polyethylene composition and to avoid bubbles formed in the formed article, because a crosslinking promoter and a free radical inhibitor have opposite effects on crosslinking, their effects may be offset if used together. Meanwhile, use of a free radical inhibitor may result in significant decrease in the degree of crosslinking and the curing (maintaining) speed of the formed article.
However, this invention intends to try a different approach of using a crosslinking agent, a crosslinking promoter and a free radical inhibitor simultaneously in a rotational molding polyethylene composition, by taking advantage of the difference in time and temperature when the crosslinking promoter and the free radical inhibitor take effect, in search of a crosslinked polyethylene composition with improved processability, so as to significantly improve crosslinking retardation time of the crosslinked polyethylene composition, and effectively prevent the crosslinked polyethylene composition from premature crosslinking and avoid formation of bubble in the form article. Meanwhile, use of a high temperature crosslinking agent in combination with a crosslinking promoter and a free radical inhibitor significantly increases the safety process temperature of the crosslinked polyethylene composition.
The crosslinked polyethylene composition comprising a crosslinking agent, a crosslinking promoter and a free radical inhibitor is especially suitable for the manufacture of large rotational molding hollow articles.